The invention relates generally to carbon dioxide dry cleaning systems, and, more particularly, to a shaft seal system for carbon dioxide dry cleaning systems that utilize rotating baskets to agitate the items being cleaned.
Due to environmental, health and safety concerns, dry cleaning systems that utilize liquid carbon dioxide as a solvent instead of perchloroethylene (xe2x80x9cpercxe2x80x9d) or petroleum-based solvents have grown in popularity. Liquid carbon dioxide is an inexpensive and unlimited natural resource and is non-toxic, non-flammable and does not produce smog. It does not damage fabrics or dissolve common dyes and exhibits solvating properties typical of more traditional solvents.
Dry cleaning systems require that liquid carbon dioxide be maintained at a temperature of approximately 50-60xc2x0 F. to enhance solvent properties. This temperature range corresponds to a pressure range of 680-700 psig. As a result, the cleaning vessel, piping and all associated components of liquid carbon dioxide dry cleaning systems must be designed to operate at high pressure.
In order to minimize the wrinkling that may occur in some materials and to better recover carbon dioxide during the gas recovery/drying cycle, it is desirable to place the items being cleaned in a basket that rotates within the cleaning vessel. The high pressure requirements of carbon dioxide systems, however, have made the provision of a rotating basket problematic. For example, conventional dry cleaning systems often use a shaft connected between the basket and an appropriate electromechanical drive system. The shaft must therefore pass through the wall of the cleaning vessel. As a result, difficulties in preventing high pressure carbon dioxide from escaping around the shaft have been encountered. Alternatives, such as rotating the basket through a magnetic coupling, have been found to be impractical. This is especially true in the case of large, commercial-sized cleaning systems.
U.S. Pat. No. 5,943,721 to Lerette et al. discloses a carbon dioxide dry cleaning system with a rotating basket and a shaft seal arrangement that attempts to overcome some of the difficulties of the prior art systems. The shaft of the Lerette et al. ""721 patent is supported by a pair of bearings that are spaced with respect to each another and the cleaning vessel. A dual-seal arrangement surrounds the shaft as it passes through the rear wall of the cleaning vessel. An annular sealant chamber is defined between the two seals and is provided with pressurized sealant fluid at a pressure approximately 500 psig below the pressure of the wash fluid in the cleaning vessel. As a result, the pressure across each of the two seals is substantially less than the pressure differential between the pressure within the cleaning vessel and atmospheric pressure.
A disadvantage of the system of the Larette et al. ""721 patent, however, is that the bearings supporting the shaft are positioned outside of the cleaning vessel and, therefore, at a significant distance from the basket. Such an arrangement causes premature bearing wear and bearing maintenance issues.
Bearings require some form of lubrication. Oil typically is used. Carbon dioxide is a very aggressive solvent of oil, however, and, as such, proper measures must be taken to ensure that bearings positioned within the cleaning vessel are properly lubricated and protected from carbon dioxide leaks. The Larette et al. ""721 patent and the prior art do not effectively address this issue.
The system of the Larette et al. ""721 patent also fails to provide an arrangement whereby leakage of carbon dioxide solvent from the cleaning vessel through the shaft seal may be easily detected and managed. Failure to control such leaks could result in the interruption of the dry cleaning operation.
Other prior art sealing arrangements include dual seals with sealant chambers in between with and without a barrier fluid and with and without pressurization. In addition, the prior art includes labyrinth seals whereby the labyrinth space between a pair of seals is filled with an inert gas at a pressure higher than the pressure in the cleaning vessel. As a result, if seal leakage occurs, a small quantity of harmless inert gas leaks into the cleaning vessel and/or the atmosphere. The space between the seals in such an arrangement, however, could not accommodate oil for lubricating bearings. More specifically, if leakage of the seal between the sealant chamber and the cleaning vessel occurred, oil would be forcibly leaked in the cleaning vessel resulting in contaminated cleaning fluid. In addition these prior art systems fail to resolve the bearing and leak detection and management issues described previously.
Accordingly, it is an object of the present invention to provide a reliable shaft seal system for a liquid carbon dioxide dry cleaning system.
It is another object of the present invention to provide a shaft seal system for a liquid carbon dioxide dry cleaning system that permits a shaft bearing to be placed close to a rotating basket and that provides proper lubrication for the bearing.
It is another object of the present invention to provide a shaft seal system that facilitates the detection of leakage.
It is still another object of the present invention to provide a shaft seal system that manages leaks.
It is still another object of the present invention to provide a shaft seal system that is easy to service and maintain.
The present invention is directed to a system for sealing a shaft that rotates a basket in the pressurized cleaning vessel of a dry cleaning system of the type that utilizes liquid carbon dioxide as a solvent. The system features a cartridge housing surrounding the shaft as it enters the cleaning vessel with a primary seal disposed between the shaft and the cartridge housing. A secondary seal is also disposed between the shaft and the cartridge housing and is spaced from the primary seal so that a primary chamber is defined therebetween. A final seal is disposed between the shaft and the cartridge housing and is spaced from the secondary seal so that a secondary chamber is also formed. The primary and secondary chambers are at atmospheric pressure when the primary and secondary seals are fully intact. Inboard and outboard bearings for supporting the shaft are positioned within the primary and secondary chambers, respectively. A thrust bearing is also positioned within the secondary chamber.
Primary and secondary tanks containing a lubricating fluid, such as oil, are connected by both their wet sides and head spaces to the primary and secondary chambers, respectively. As a result, the inboard, outboard and thrust bearings are lubricated. Solvent leaking from the cleaning vessel across the primary seal into the primary chamber is vaporized and routed to the head space of the primary tank thus reducing the pressure differential across the primary seal. Similarly, leaks of solvent across the secondary seal are routed to the head space of the secondary tank also reducing the differential pressure across the seal. A pressure switch having a low pressure setting is in communication with the secondary chamber and a signal device so that the signal device is activated when the pressure within the secondary chamber exceeds the pressure setting of the pressure switch. As such, the signal device provides an indication of leakage across the secondary seal. Pressure gauges are also in communication with the chambers and indicate the presence and severity of leaks. Lines featuring check valves are in communication between the head spaces of the primary and secondary tanks and the cleaning vessel so that solvent vapor leaked into the primary and secondary tanks and stored there until the cycle ends may be returned to the cleaning vessel when it is depressurized. This also is to prevent oil from being forced into the cleaning chamber. A vacuum pump removes the air from the cleaning vessel at the beginning of each cycle. As pressure enhanced seals are effective in only one direction, without the check valve, oil could be drawn into the cleaning vessel by action of the vacuum pump.
The following detailed description of embodiments of the invention, taken in conjunction with the appended claims and accompanying drawings, provide a more complete understanding of the nature and scope of the invention.